1. Field of the Invention
The present invention relates to a condenser microphone configured to have a plurality of condenser microphone units connected in series to improved output sensitivity, and allow balanced output of positive and negative audio signals from the condenser microphone units connected in series.
2. Description of the Related Art
A condenser microphone generates an audio signal based on a change in capacitance between a diaphragm and a fixed electrode which are opposed to each other.
In this configuration, a condenser microphone unit is disposed with the diaphragm opposing to the fixed electrode, and the condenser microphone unit has a capacitance of approximately several ten picofarads, and an output impedance is considerably high. Therefore, the audio signal is extracted using an impedance converter using for example a field effect transistor (FET).
Various devices for improving the output sensitivity of the condenser microphone have been proposed conventionally, a condenser microphone using a plurality of condenser microphone units connected in series to improve output sensitivity has been proposed by an applicant of the present invention, and disclosed in JP 5201598 B2.
FIG. 5 is a block diagram illustrating a configuration of a condenser microphone disclosed in JP 5201598 B2.
The configuration illustrated in FIG. 5 includes first to fourth condenser microphone units U1 to U4, and each of the condenser microphone units U1 to U4 constitutes an electret condenser microphone unit including a dielectric electret film on any of a fixed electrode or diaphragm.
The condenser microphone illustrated in FIG. 5 is configured so that audio signals obtained from the first and second condenser microphone units U1 and U2 are added in series in the same phase, and audio signals obtained from the third and fourth condenser microphone units U3 and U4 are also added in series in the same phase.
That is, a diaphragm 1a of the first condenser microphone unit U1 is connected to a ground line as a reference potential point of a circuit, and the fixed electrode 2a opposed to the diaphragm 1a is connected to a first impedance converter 3a. Therefore, the audio signal is generated by the impedance converter 3a based on a change in capacitance between the diaphragm 1a and the fixed electrode 2a of the condenser microphone unit U1.
The audio signal obtained from the first condenser microphone unit U1, generated by the first impedance converter 3a, is supplied to the diaphragm 1b constituting the second condenser microphone unit U2. The fixed electrode 2b opposed to the diaphragm 1b is connected to the second impedance converter 3b, and the audio signal is generated by the second impedance converter 3b based on a change in capacitance between the diaphragm 1b and the fixed electrode 2b of the condenser microphone unit U2.
In this configuration, the second condenser microphone unit U2 is configured so that the audio signal obtained from the first condenser microphone unit U1 is applied to the diaphragm 1b, and audio signals obtained from the first and second condenser microphone units U1 and U2 are added in the same phase and output from an output terminal Out (+) of the second impedance converter 3b. 
Therefore, the output sensitivity as the condenser microphone can be doubled.
While, the third and fourth condenser microphone units U3 and U4 also include a third impedance converter 3c and a fourth impedance converter 3d, respectively, and are operated so that audio signals obtained from the third and fourth condenser microphone units U3 and U4 are added in the same phase, similarly to the operation of the first and second condenser microphone units U1 and U2 having been described the above.
However, in the third condenser microphone unit U3, the fixed electrode 2c is connected to the ground line, and the diaphragm 1c is connected to the third impedance converter 3c. Further, output of the third impedance converter 3c is supplied to the fixed electrode 2d of the fourth condenser microphone unit U4, and the diaphragm 1d is connected to the fourth impedance converter 3d. 
Accordingly, audio signals of opposite phase are output with balance from the output terminal Out (+) of the second impedance converter 3b and an output terminal Out (−) of the fourth impedance converter 3d. 
A condenser microphone disclosed in JP 5201598 B2 includes impedance converters corresponding to the condenser microphone units connected in series as described the above, respectively.
The impedance converters employ active elements such as an FET, respectively. Accordingly, each of the impedance converters requires a circuit configuration for driving the active element, and further requires a configuration for supplying operation power (DC power supply Vcc) for each of the active elements. Therefore, the whole circuit structure of the condenser microphone is complicated, and inevitably results in a high cost.
Further, the operation power supply uses a limited power supply such as a known phantom power supply or a battery, and when a plurality of impedance converters using active elements such as an FET is used, each of the impedance converters requires a drive current. As a result, even if for example the phantom power supply is used, the condenser microphone has such a problem that a drop in voltage of the operation power supply for the impedance converter is increased, a maximum output level of the condenser microphone is also limited, and it is difficult to increase a dynamic range.
Further, In order to obtain the balanced positive and negative audio signals, the condenser microphone unit needs to be divided into two for a positive phase signal and a negative phase signal, and the condenser microphone unit is required to have the same number between the positive phase signal and the negative phase signal. In addition, since the condenser microphone unit is divided into the two for the positive phase signal and the negative phase signal, when a difference is made in overall sensitivity between signal transmission paths each including a plurality of impedance converters, the audio signal is output while having an unbalanced level between the positive phase signal and the negative phase signal, and the quality of the audio signal is inevitably deteriorated.